There are numerous occasions, as in the practice of medicine and related arts, in which it is desirable to employ materials having a low coefficient of friction. For example, articles and devices such as contact lenses, catheters, peristaltic pumps, condoms, arteriovenous shunts, gastroenteric feed tubes and endotracheal tubes should be made of materials which have slippery surfaces when placed in contact with bodily fluids to facilitate insertion and removal of the articles from the body.
A class of materials which exhibit low friction properties, to a marked degree, are the hydrophilic polyurethane polymers described in U.S. Pat. Nos. 3,822,238 and 3,975,350. In the presence of an aqueous medium, these polymers absorb water with concomitant increase in volume and formation of a stable, water insoluble hydrogel distinguished by its very low coefficient of friction. Such polymers have found extensive usage in the manufacture of various types of body implants. Owing to the extremely slippery nature of the hydrogel or hydrated form of the polymer from which they are fabricated, the implants are readily inserted and removed from body tissues with minimal discomfort to the patient. Other important uses for the polyurethane polymers based on their low coefficient of friction are drag resistant surfaces such as coatings for boat hulls and the inner linings of conduits.
Another class of hydrophilic polyurethanes that form insoluble hydrogels is disclosed in U.S. Pat. Nos. 4,156,066 and 4,156,067. These polyurethanes are characterized by the presence of a lactone group in the polymer backbone. The lactone may be opened by hydrolytic cleavage to form a carboxylic acid which renders the polymer soluble in alkaline medium. Other specialized hydrophilic polyurethanes are the polyurethane diacrylates of U.S. Pat. No. 4,359,558, the polyurethane quaternary ammonium salts of U.S. Pat. No. 4,451,635 and the polyurethane polymers prepared from specific mixtures of alkylene glycols as described in commonly assigned U.S. patent application Ser. No. 879,156 filed June 6, 1986 now abandoned.
All of the above-cited U.S. patents and application are incorporated herein by reference as representative of the hydrophilic polyurethane polymers treatable in accordance with the present invention.
The hydrophilic character of the polyurethane polymers described in the cited patents is attributable to the presence of polar sites in the polymer backbone. In general, these are controlled by selecting the proper active hydrogen resin to be reacted with the polyisocyanate when carrying out the polyurethane synthesis. The polymers may be thermoplastic or thermosetting, depending on the degree of cross-linking. Difunctional reactants tend to give thermoplastic polymers whereas reactants of higher functionality tend to give thermoset polymers. By following the teachings of the patents, reactants can be selected to produce hydrophilic polyurethane polymers suitable for a variety of applications.